1. Field of the Invention
The present invention relates to a small-size reflection type liquid crystal display device with low power consumption. More particularly, the present invention relates to a reflection type liquid crystal display device capable of providing a bright color display, and a reflection type liquid crystal display device provided with an auxiliary light system.
2. Description of the Related Art
Conventionally, liquid crystal display devices have been used as displays for information terminals. Such liquid crystal display devices are categorized into two types: a transmission type liquid crystal display device capable of providing a bright display by using a backlight system as a light source; and a reflection type liquid crystal display device providing a display by reflecting ambient light without the use of a backlight system as a light source.
Although realizing bright display, the transmission type liquid crystal display device is heavy and consumes a large amount of power since it includes the backlight. In contrast, the reflection type liquid crystal display device, which does not require a backlight, is light in weight and consumes only a small amount of power. With such advantages, the reflection type liquid crystal display devices are especially useful as displays for portable information terminals.
Conventionally, as the reflection type liquid crystal display devices, those which provide a monochrome display in the twisted nematic (TN) mode, the super-twisted nematic (STN) mode, and the like have been put into practical use. For realizing color display, the reflection type liquid crystal display device provided with RGB color micro-filters for each pixel has been developed.
The color reflection type liquid crystal display devices may be realized, not only by using a color filter, but also by utilizing light interference due to the birefringence effect of the liquid crystal, or by utilizing multi-interference between multi-layer films. However, in the former method utilizing light interference, since the retardation varies depending on the temperature and the angle of vision, the color hue itself may vary. In such a case, even black and white display is difficult. The latter method utilizing multi-interference is disadvantages in cost and production efficiency compared with the method using a color filter.
In general, in a reflection type liquid crystal display device provided with a color filter, ambient light passes through the color filter twice. When the same color filter used for a transmission type liquid crystal display device is used for the reflection type liquid crystal display device, there is an extreme decrease in the light transmittance, resulting in a dark display. To avoid this problem, a color filter with a high light transmittance should be used for the reflection type liquid crystal display device.
As a specific example, FIG. 10 shows the wavelength versus transmittance characteristics of red color filters used for the transmission type and reflection type liquid crystal display devices as curves 61 and 62, respectively.
As is observed from FIG. 10, while the red color filter used for the transmission type liquid crystal display device effectively absorbs light within the wavelength range of 580 nm or less, the red color filter used for the reflection type liquid crystal display device has a transmittance higher than that for the transmission type liquid crystal display device for light within the wavelength range of 580 nm or less.
In such a conventional color filter used for the reflection type liquid crystal display device, in order to secure the brightness, the light transmittance of the color filter for the wavelength range of 580 nm or less was merely increased. Actually, increasing the light transmittance causes a minute change in color hue. This change in color hue occurs in any of red, green, and blue color filters: Red changes toward orange, green changes toward yellowish green, and blue changes toward cyan. A white display obtained by the additive color mixture among red, green, and blue also changes to a yellow or bluish display. The change of red toward orange particularly reduces the visibility.
In full-color display, the color balance is lost as the color hues of red, green, and blue change. Therefore, fore, a desired display color cannot be reproduced, resulting in reducing the display quality.
The reason why the color hue changes will be described, taking the red color filter as an example.
In a red color filter having the characteristics represented by the curve 62 in FIG. 10, in order to increase the light transmittance, the transmittances for light in the green wavelength range (around 540 nm) and the blue wavelength range (around 450 nm) have been increased substantially equally. Accordingly, the color hue is slightly shifted toward green and blue. The shift toward blue hardly causes a problem. However, the shift toward green causes red to be recognized as yellowish red. As a result, a color hue shifted toward orange is recognized.
The change in color hue described above is especially observed in red and white.
Another problem is that, as the light transmittance of the color filter is made higher, the color purity becomes lower, resulting in a pale display as a whole and thus a reduced chroma. Accordingly, the light transmittance of the color filter must be selected in consideration of the color purity.
In reflection type liquid crystal display devices for mainly displaying characters and diagrams, such as a portable personal computer, a wordprocessor, a game machine, and the like, the brightness of the display is sometimes regarded more important than in devices for displaying natural images. In such a case, the optimization of the brightness and the chroma (color purity) is different from that performed for the devices for displaying natural images. That is, in such a case, it is desirable to obtain a brighter display even though the chroma is reduced to the extent where the display color is barely recognizable.
In recent years, a new reflection type liquid crystal display device has been developed, which functions as the normal reflection type under the environment where ambient light is prevalent, such as outdoors and near a window indoors, but uses an auxiliary light to supplement reduced brightness of ambient light under the environment where the amount of ambient light is reduced.
In such a reflection type liquid crystal display device, the chroma can be improved by lighting the auxiliary light compared with the conventional reflection type liquid crystal display device.
However, the auxiliary light only supplies an amount of light sufficient enough to supplement the shortage of ambient light, not emitting intense light as is done by a backlight of the transmission type liquid crystal display device. Accordingly, the brightness is considerably reduced if a color filter with a high color purity (such as the color filter used for the transmission type liquid crystal display device) is used for the reflection type liquid crystal display device provided with the auxiliary light.